1. Field of the Invention
The present invention relates to a radiation image detection method and a radiation image detection system for reading out a radiation image recorded in a radiation image detector by irradiating the radiation image detector with readout light. The radiation image recorded in the radiation image detector is an image recorded by storing electric charge, based on the dose of radiation, generated by irradiation with radiation carrying a radiation image.
2. Description of the Related Art
Conventionally, a radiation image detection system which records radiation images using a radiation image detector, and which reads out the radiation images recorded in the radiation image detector as electric signals has been widely utilized in medical radiography or the like. Further, various kinds of such radiation image detection system have been proposed. In the radiation image detector, a radiation image of a subject is detected by storing electric charge corresponding to the dose of radiation, such as X-rays, transmitted through the subject in a charge storage portion.
As an example of the radiation image detector which is used in the radiation image detection system, as described above, a radiation image detector is disclosed in U.S. Pat. No. 6,268,614. In the radiation image detector disclosed in U.S. Pat. No. 6,268,614, a first electrode layer, a photoconductive layer for recording, a charge transport layer (an electric charge transport layer), a photoconductive layer for readout and a second electrode layer are superposed one on another in this order. The first electrode layer is a layer which transmits radiation. The photoconductive layer for recording is a layer which generates electric charge by irradiation with radiation. The charge transport layer is a layer which acts as an insulator against latent image charge, and which acts as a conductor of transport electric charge, of which the polarity is opposite to that of the latent image charge. The photoconductive layer for readout is a layer which generates electric charge by irradiation with readout light. The second electrode layer is a layer, in which linear electrodes are arranged in parallel. In the radiation image detection system using the radiation image detector, as described above, the radiation image detector is irradiated with radiation from the first electrode layer side of the radiation image detector while voltage is applied to the first electrode layer and the second electrode layer. Then, electric charge corresponding to the dose of the radiation, with which the radiation image detector is irradiated, is generated in the photoconductive layer for recording. In the electric charge generated in the photoconductive layer for recording, an electric charge of one of the polarities is combined with an electric charge charged in the first electrode layer. At the same time, in the electric charge generated in the photoconductive layer for recording, an electric charge of the other polarity is stored, as latent image charge, in a charge storage portion formed at the interface between the photoconductive layer for recording and the charge transport layer. Accordingly, a radiation image is recorded. Then, the radiation image detector is irradiated with readout light from the second electrode layer side. The readout light is transmitted through the second electrode layer, and the photoconductive layer for readout is irradiated with the readout light. When the photoconductive layer for readout is irradiated with the readout light, electric charge is generated in the photoconductive layer for readout. In the electric charge generated in the photoconductive layer for readout, an electric charge of one of the polarities is combined with the latent image charge stored in the charge storage portion. At the same time, an electric charge of the other polarity is detected by an electric current detection amplifier connected to the linear electrode. Accordingly, the radiation image is detected as electric signals.
In U.S. Pat. No. 6,268,614, a-Se is used as a material for the photoconductive layer for recording. Further, a radiation image detector, in which a crystallization prevention layer is provided between the photoconductive layer for recording and the first electrode layer, has been proposed to prevent crystallization of a-Se (U.S. Pat. No. 6,770,901).
However, in recent years, it was found out that there are some cases in which electric charge remains in a portion of the photoconductive layer for recording which is in the vicinity of the first electrode layer after a radiation image is read out from a radiation image detector.
For example, in the radiation image detector disclosed in U.S. Pat. No. 6,268,614, when a radiation image is recorded, the movement of an electric charge, of which the polarity is opposite to that of the latent image charge, in the electric charge generated in the photoconductive layer for recording is prevented at the interface between the photoconductive layer for recording and the first electrode layer. Therefore, the electric charge remains at a portion of the photoconductive layer for recording which is in the vicinity of the first electrode layer in some cases. For example, if the latent image charge is a negative electric charge, the electric charge, of which the polarity is opposite to that of the latent image charge, is a positive electric charge. Particularly, when a crystallization prevention film is provided between the photoconductive layer for recording and the first electrode layer, there is a tendency that the movement of more electric charge is prevented and more electric charge remains. Such electric charge which remains in the vicinity of the first electrode layer is not substantially read out or erased in ordinary readout operations. Therefore, when a next radiation image is recorded, there is a risk that the image quality of the radiation image deteriorates because the next radiation image is recorded without erasing the electric charge remaining in the vicinity of the first electrode.
It is well known that generally, irradiation with erasing light, particularly, irradiation with erasing light, of which the wavelength is within the bandwidth of wavelengths of blue light, or irradiation with erasing light, of which the wavelength is within the bandwidth of wavelengths of green light, is effective to erase the remaining electric charge. In U.S. Pat. No. 6,268,614, the radiation image detector is irradiated with erasing light, of which the wavelength is within the bandwidth of wavelengths of blue light, or with erasing light, of which the wavelength is within the bandwidth of wavelengths of green light, from the second electrode layer side to erase the remaining electric charge in the vicinity of the charge storage portion. However, the light with a wavelength within such bandwidths is absorbed in the photoconductive layer. Therefore, when the radiation image detector is irradiated with such erasing light from the second electrode layer side, the erasing light does not reach the vicinity of the first electrode layer. Hence, the remaining electric charge in the vicinity of the first electrode layer is not substantially erased by performing conventional erasing operations, in which the radiation image detector is irradiated with the erasing light from the second electrode layer side.
Meanwhile, if the radiation image detector is irradiated with erasing light from the first electrode side, the remaining electric charge is excited and the movement of the electric charge is accelerated. Hence, the remaining electric charge can be erased. However, there is a problem that if a light source unit for erasing light is provided on the first electric side, the thickness of the radiation image detector becomes thick.